As disclosed in Federal Aviation Regulation Part 25 Sec. 25.981, the US Federal Aviation Administration promulgated regulation about the firing prevention of a fuel tank. According to this regulation, it is necessary to take measures so as for any ignition source not to generate in the fuel tank even in either of a case of a single failure, a case of a combination of the single trouble and a latent failed state that the relation to the single failure is not low, and a case of a combination of failures having possibilities which are not very low.